In this new research, wideband velocity estimation techniques developed by the principal investigator, using the return from an acoustic signal, will be applied to the problem of the estimation of the blood velocity profile. Due to the use of narrowband estimators with limited spatial and velocity resolution, ultrasonic instruments with a single sample volume cannot currently provide a reliable estimate of velocity spread in small spatial regions, thus limiting the detection of stenotic arterial lesions which encompass less than 50% of the vessel diameter. In addition, current instruments are limited in their detection of slow flow through small vessels, due to the requirement for the transmission of a narrowband signal, and the insensitivity of current clutter rejection, or wall filters. It is anticipated that the new wideband estimation approach will establish a more rigorous limiting performance in blood flow estimation, and provide an improved methodology for the study of the physiology of the circulatory system, as well as the detection and quantification of heart and vascular abnormalities. Using a wideband transmitted signal, optimal estimation of mean velocity and velocity spread within each small spatial region will be studied for these long observation intervals, first through a theoretical development. This will include a theoretical evaluation of the fundamental limitations on the detection of disturbed flow, and the fundamental limitations on the detection of low velocity flow through small vessels. This research will include the use of alternative signaling schemes, as well as proposed three dimensional spatial velocity plots. Experimental data will then be acquired and analyzed in order to complement the theoretical results. This data will be acquired from phantom vessels with varying degrees of stenosis, variations in vessel size, and variations in vessel orientation. In addition to a potential improvement in the understanding of the behavior of small volumes of the blood scattering medium, the defined research project could produce several important improvements in flow estimation. First, an improved sensitivity to low velocity flow through small vessels could significantly improve the detection of small vascularized tumors. Second, the principal investigator has previously shown that the new wideband strategies are very sensitive to the width of the illuminated velocity distribution, and can therefore significantly improve the estimation of turbulence, which is extremely important in assessment of pathology. In addition, new wideband techniques can eliminate aliasing, which affects narrowband techniques and therefore current instruments.